In several communications networks, trunk lines are reaching the limit of data-transmission capacity. Wave-length-division multiplexing (WDM) is a cost-effective means to boost capacity without the need to install new fiber or upgrade bandwidth per channel. In WDM systems, data are transmitted over multiple, closely spaced wavelengths, or channels, increasing the capacity of a single transmission line many times over; WDM systems under test feature from four (4) to forty (40) channels.
In communications, the channel crosstalk must be kept below 25 Db, which requires the wavelength drift of transmitters for dense WDM systems to be small compared to channel separation. The suggested grid for channel wavelengths is 1552.52 nm+-n*0.8 nm, where n is the channel number (1, 2, 3 and so forth). Without wavelength locking, WDM system designers must sift through lasers to find sources with wavelengths matching system-channel definitions. Currently, WDM laser transmitters can be specified to within 0.4 nm, with a wavelength stability of 0.02 nm/year over 20 years. Although this is acceptable for a channel spacing of 1.6 nm, it is certainly not appropriate for a channel spacing of 0.8 nm, unless transmitters are periodically serviced, an undesirable scenario.
Single-mode diode lasers such as distributed-feedback lasers are commonly operated using thermoelectric coolers for temperature, and, hence, wavelength stabilization. While this technique is adequate over short periods, over the long term laser emission wavelength tends to drift over time.
Conventional wavelength lockers monitor and control the wavelength of light produced by a light source. Conventional light sources are lasers, often distributed feedback Bragg reflective ("DFB") lasers. A DFB laser is typically tuned to produce light of a predetermined wavelength. However, as a DFB laser is used, the current generated in the cavity changes the resonant characteristics of the cavity. Consequently, the wavelength of the light produced by a DFB laser drifts from the predetermined wavelength as the DFB laser is used.
In conventional wavelength lockers, light from the DFB laser is transmitted to a collimator and travels down a fiber. Conventional systems monitor the wavelength of the incoming light by transmitting the beam to a spectrum analyzer. The spectrum analyzer determines the wavelengths which comprise the beam of light. The spectrum analyzer transfers the information on the wavelength to a feedback system. The feedback system uses this information to change the temperature of the DFB laser to compensate for any drift in the wavelength of the light from the predetermined value.
Optimizing signal-to-noise ratios throughout the system directly affects performance and can eliminate the use of unnecessary amplifiers. Therefore, WDM systems are designed, installed, verified and monitored using the following measurements:
Carrier wavelengths and powers
In any optical transmission system the wavelength and power of the laser transmitter is important. This is particularly true for WDM systems, in which accurate wavelengths are necessary to avoid interference with adjacent channels and accurate amplitude levels are required to account for loss and amplifier efficiency at different wavelengths. Measuring carrier wavelengths optimizes for system components. Measuring carrier powers optimizes for transmitter reliability and system performance.
Channel spacing
In a WDM system it is critical that the difference between any two channels (the channel spacing) be adhered to. Filters are used to ensure that optical transmitters operate only within their designated channels. Measuring this fundamental specification enables optimization for system components and performance.
Flatness
The relative power levels between channels throughout the WDM system are referred to as flatness. In some systems, to account for the gain variations of the optical amplifiers at different wavelengths, carrier levels are purposely offset from each other. By measuring the relative differences between carrier levels in the system, the flatness can be determined. Measurements of flatness can help optimize for system components and performance.
Drift
Drift in carrier wavelength is the change in wavelength over time caused by temperature, laser instability, degradation, etc. If a transmitter drifts in a system, it may approach the edge of the channel filters, reducing transmitted power. The result will be increased system error and, eventually, channel failure. Measuring the drift of the optical carriers in a WDM system will provide an indication of overall system stability and verification of performance.
Signal-to-noise ratio
The signal-to-noise ratio for each channel is one of the most important measurements in an optical transmission system. During a signal-to-noise ratio measurement, the absolute power of the carrier (in dBm) is compared with the absolute power of the noise (at the carrier wavelength). Since noise power varies with measurement bandwidth (a wide bandwidth allows more noise to the detector than a narrow bandwidth), the noise bandwidth used during the measurement must be accounted for. Typically, this means normalizing the noise power measurement to a known bandwidth (such as 1 Hz or the analog bandwidth as defined in the SONET/SDH standards). Measuring this parameter enables optimization for system performance.
Although the conventional systems are capable of determining and controlling the wavelength of light produced by a DFB laser, those with ordinary skill in the art will realize that the spectrum analyzer disrupts the beam. In order for the spectrum analyzer to determine the wavelengths comprising the incoming light, the spectrum analyzer is given access to the entire beam. In addition, the spectrum analyzer is bulky and expensive. Consequently, any conventional system for monitoring controlling the wavelength of light in a wavelength locker will be costly and relatively large.
Accordingly, what is needed is a system and method for providing a wavelength locker in which the wavelength of the transmitted light is monitored without significant interruption of the light. In addition, it would be beneficial if the system was less bulky, cheaper and easier to manufacture. The present invention addresses such a need.